As well-known in the art, there are many different biometric authentication methods such as a face authentication method, a fingerprint authentication method, an iris authentication method, a blood vessel authentication method and the like.
Among them, the face authentication method makes use of a lighting fixture and a camera for acquiring a face image and an image display device for displaying a face of a living body to be authenticated imaged by the camera.
In this face authentication method, the distance between the camera and the living body to be authenticated is an important factor for acquiring a face image effective for face authentication.
In a face authentication device according to a related art, a guide line is displayed on a screen of an image display device in order to guide a living body to be authenticated to a predetermined recognition distance. If a taken face image is displayed on the screen, a user adjusts the distance between the user and the camera so that a contour line of a face image displayed on the screen overlaps with the guide line as far as possible. Moreover, a message which instructs the user to come closer or go farther is visually displayed or verbally notified in order to guide a living body to be authenticated to a predetermined recognition distance.
As mentioned above, according to the related art, a biometric subject should make an effort to align his or her face with the guide line displayed on the screen. This poses a problem in that the biometric subject may feel obsessive.
Even if the face is aligned with the guide line, the distance from the camera to the biometric subject may vary depending on the size of the face of the biometric subject. In photography, the distance in front of and behind the subject that appears to be in focus is referred to as a depth of field (DOF). In the case of using a camera having a small depth of field, even if the face of a biometric subject is aligned with a guide line, optical blurring may occur in a face image due to the difference in the distance between the biometric subject and the camera.
Thus, according to the related art, face recognition performance may be reduced due to the failure of face recognition attributable to optical blurring or the like. The failure of face recognition may cause inconvenience to the biometric subject.
In the meantime, if an iris having a large aperture value is used in order to increase the depth of field, the amount of light accumulated per unit time in an image sensor of a camera decreases. This poses a problem in that a face image acquired becomes darker. If the exposure time of a camera is increased in order to solve this problem, motion blurring may be generated due to the motion of a biometric subject. After all, in order to solve the aforementioned problem, it is necessary to increase the number of lightings or to use a strong lighting.
However, the increase in the number of lightings may lead to an increase in the size of a face recognition device. Use of a strong lighting may adversely affect the eyes of a biometric subject and may cause great inconvenience to the biometric subject.
Accordingly, in order to prevent a decrease in the face recognition performance, it is necessary to suitably set the number of lightings, the brightness of lightings, the aperture value of a camera, and the exposure of a camera. It is preferred that the face of a user is positioned within a depth-of-field range which is a predetermined recognition distance that satisfies such a condition. As a consequence, it is most preferable that a biometric subject is positioned at a predetermined recognition distance.